I think this is new. It’s certainly causing lots of confusion.
All of my Strava rides were getting auto-posted to Facebook. This was unwanted.
The problem is Strava has a cryptic poorly implemented auto-share toggle setting that’s not part of Strava settings. A deluge of customer complaints forced Strava to break it’s no-documentation rule:
Directly after uploading an activity, you will see the activity edit page before clicking "Save & View." You can share activities to Facebook by flipping ON the Facebook toggle for any activities you wish to share (button turns blue). Keep in mind that this toggle will turn on automatic sharing. In other words, all future activities will be shared as long as this button is left on.
Take a look at the screenshot on that web site. It took me a while to find the Facebook icon. If it’s blue then auto-share is on and will stay on. To turn it off create a fake ride (walk around the house) then click the “Flag” icon to save it then hunt for a blue Facebook icon at bottom right of the “save” screen. Toggle it off. It will keep that state.
When auto-share is off you can still post to Facebook, you have to open the saved activity then click on it again and then click share.